In a wireless system whose multiple-access scheme is based on carrier sense multiple access/collision avoidance (CSMA/CA) in accordance with IEEE 802 standards, (e.g., IEEE 802.11), WTRUs determine when to attempt receiving and transmitting packets based on the strength of received signals transmitted from neighboring WTRUs utilizing the same channel. By avoiding transmitting simultaneously, multiple WTRUs can avoid mutually interfering with each other while sharing the same channel.
A few key parameters, such as an energy detect threshold (EDT) and a defer threshold (DT), are typically used by a WTRU in determining when it is possible to transmit or receive a packet. The EDT represents the smallest received signal power for which reception of a packet will be attempted. The DT represents the smallest received signal power for which transmission of a packet by the WTRU will be deferred. The DT value may or may not be the same as the EDT value.
Although the basic concept of CSMA/CA is straightforward, there are certain situations where it results in undue reduction of capacity with respect to the maximum capacity that could be obtained. For instance, there are cases where two transmitters, (communicating with different receivers), could transmit simultaneously without interfering significantly with each other, but they do not do so because they hear each other above the values of their respective EDT and/or DT. This results in a waste of capacity which could be avoided if one of the following actions would take place:
a) The WTRUs reduce their transmission powers, so that the WTRUs cannot hear each other above the EDT, (and/or DT), but can still communicate at the optimum data rate to their respective counterparts;
b) the WTRUs raise their EDT, (and/or DT), so that the signals they receive from each other fall below this threshold; or
c) a combination of a) and b).
The techniques by which a node or a system determines and sets the above-mentioned parameters, (transmission power, EDT, DT), will be hereinafter referred to as deferral management. The appropriate setting of these parameters would be facilitated if the WTRUs would know their path losses between each other, as well as the EDT or DT values used by other WTRUs. This knowledge would allow a WTRU, (or another node, such as an access point (AP) which controls some of the parameters of the WTRU), to predict if transmitting at a certain level would result in a received signal being above or below the EDT for a given neighboring WTRU. This knowledge could thereafter be used in algorithms aimed at optimizing capacity.
Obtaining path loss estimates may also be useful for positioning purposes. Using the knowledge of the set of path losses between pairs of WTRUs, as well as the knowledge of the position of several fixed WTRUs, (typically APs or mesh points (MPs in a mesh network), it is possible to estimate the location of a WTRU. This estimation could be performed by a node, (which may or may not be a WTRU), collecting the path loss estimates from one or several different WTRUs.
Therefore, a method by which WTRUs could reliably estimate their path loss to other WTRUs is desired.
Problem to be Solved
One straightforward manner in which a first WTRU could estimate the path loss between itself and a second WTRU that is neighboring the first WTRU would be for the first WTRU to subtract the received power it perceives from the transmission power used by the second WTRU. This assumes that the first WTRU is capable of determining the power at which the second WTRU transmits a signal that the first WTRU measured. The following are reasons why this is not possible in the current state of WLAN technology:
1) The setting of transmission power by APs, WTRUs, and MPs in state-of-the-art WLANs is proprietary and not communicated to other WTRUs. For example, the existing 802.11h/802.11k transmission power control (TPC) mechanism only imposes a maximum transmission power setting for the basic service set (BSS) that is not to be exceeded for regulatory purposes. It should be understood that any WTRU may vary transmission power below that maximum imposed limit, without communicating its internal proprietary decisions to other WTRUs, APs, or MPs.
2) The transmission power that each WTRU uses can vary on a packet basis. This implies that, in order for the first WTRU to estimate the path loss separating it from the second WTRU, the first WTRU would have to receive a packet from the second WTRU whereby the power at which the packet was transmitted would be indicated inside the packet. Using the same aforementioned example, the only signaling that allows such a packet to be transmitted is the 802.11h/802.11k TPC request/TPC report frame exchange mechanism which requires a dedicated two-way signaling exchange and cannot be set periodically. Moreover, the TPC requests/reports cannot be sent to WTRUs that are outside the BSS. In scenarios where WTRUs aim at adjusting their transmission power and DT/EDT parameters such that they would increase the capacity of the system without creating a hidden node inside and across BSSs in the system, the knowledge of the path loss between WTRUs from a different WTRU is often desired.
3) In many scenarios, the first WTRU may not hear the second WTRU because the second WTRU transmits at a lower power than its maximum power. This might be the case if the second WTRU uses battery management techniques or deferral management techniques. It should be noted that in scenarios where a WTRU aims at adjusting its deferral management parameters, it may be desirable to estimate the path loss of WTRUs even if it cannot hear them. There is currently no way for a WLAN node to dictate to another WLAN node to increase its transmission power or even to mandate the use of a specific transmission power other than specifying the maximum power that can be used. This also applies to the TPC request/TPC report mentioned above.
The above-mentioned issues regarding the mechanisms, (or in the lack of mechanisms), supporting path loss estimation between WTRUs pose several problems. In some cases, thee mechanisms force WTRUs to assume a value for the transmission power of the neighboring WTRU, which is not very accurate given the variations among different WTRU manufacturers and the possibility that a neighboring WTRU uses a transmission power lower than the maximum. In other cases, the mechanisms can completely prevent any path loss estimation between two WTRUs from being performed.